For purposes of RTS fares (as well as other tier points, etc.) BA does not treat IST as being "Middle East", however, a couple years ago, BA expanded their restrictions with regard to RTW fares to permit only one R/T to a variety of places, including the Middle East, but also "deep" Europe. You can fly LHR-IST-LHR, but since you are doing a CAI-LHR, you will have to pay for 2 more segments.

As to the backtracking rule, since you are flying JFK-HKG and then going on to SWP, the only way to get back to Europe is via a same-numbered flight (e.g. SYD-BKK-LHR, SYD-SIN-LHR).

Hope this helps.**